DON'T MERGE: Fix cyclic imports

[dhermes]

@tseaver this is a first stab and I want to discuss / see what you think.

The imports are easy to resolve in the storage package but not so in the datastore package. See the commit messages for more details.

[tseaver]

I see what you mean about the datastore side. I think we should break this PR up into three parts:

• the storage bits, which look "normal" (3148468, minus the pylintrc changes).
• some factoring of the datastore side (based on 0a72dc4, but something needs rearranging).
• the pylintrc changes from 3148468 (enabling cycle detection).

[tseaver]

One big problem is the 'Dataset.entity' and 'Dataset.query' methods. They create the biggest cycle, and they are API smells. E.g., instead of:

query = dataset.query(kind='Kind')

the user can as easily type:

query = Query('Kind', dataset)

Likewise for entities:

entity = dataset.entity('Kind')

could as easily be:

entity = Entity(dataset, 'Kind')

I will submit a separate PR removing those two methods and updating the docs.